Catalysts based on these elements find wide application in the processing of hydrocarbon feed stocks. They may be employed in treating material derived from other fossil fuels such as coal as well as from petroleum sources.
Most hydrodesulfurization and hydrotreatment catalyst contain cobalt and molybdenum deposited on an alumina carrier. Such catalysts have also been employed for conducting methanation, production of hydrocarbon by the Fischer-Tropsch synthesis, denitrogenation, hydroforming, hydrocracking, coal liquefaction and the water gas shift reaction. In some formulations nickel and molybdenum have been employed as well as nickel and tungsten, and in some cases other supports containing such materials as silica. Vanadium oxide catalysts supported on various carriers have also been found to be active in promoting the above types of hydrocarbon conversions.
After having been charged into an industrial reactor the catalysts are often activated by a sulfur-containing reducing atmosphere and remain in contact with sulfur and reducing substances during their entire life. There is evidence of considerable reaction of the transition metals in these catalysts with sulfur. However, the various forms of lattice and surface sulfur present on the working catalysts, and their involvement in the actual reactions such as hydrodesulfurization is not entirely understood. A considerable proportion of the oxygen bound to the transition metals, however, remains uncoverted to sulfide and it has been suggested in the case of molybdenum that a binding to both oxygen and sulfur is required for optimum performances.
In some cases the transition metals are employed alone without addition of other materials described as supports and promoters. In the past a method of preparing molybdenum and tungsten sulfide catalysts by decomposition of thiosalts has been employed. For this purpose the thiosalts are first prepared by precipitation from ammonium salt of the acid of the molybdenum or tungsten by hydrogen sulfide. The thiosalts are then decomposed in an atmosphere of hydrogen. See Kurtak et al U.S. Pat. Nos. 3,764,649 and 3,876,755. There is some evidence that in the course of this thermal decomposition the sulfides MoS.sub.3 or WS.sub.3 are first formed and that they then decompose to a non-stoichiometric sulfide containing close to two sulfur atoms, i.e., MoS.sub.2 or WS.sub.2. Thus, in the formation of these catalysts the active compound is formed by removal of excess sulfur as contrasted to the procedure in which one starts with oxide which is later converted to sulfide. In the latter case it appears that the oxide form MoO.sub.3 is the initial stable oxide form and that this is converted to the dioxide in a reducing atmosphere before being converted to the corresponding disulfide. Catalysts based on transition metal compositions and involving supports can also be prepared from the thiosalts. In use, a portion of the sulfide form is converted to oxide when one starts initially from a pure sulfide type catalyst.
In catalysts containing vanadium the stable higher oxidation state corresponds to a pentavalent form V.sub.2 O.sub.5 instead of MoO.sub.3 and WO.sub.3. The sulfide V.sub.2 S.sub.5, analogous to the unstable MoS.sub.3 and WS.sub.3, is also unstable and can be decomposed to V.sub.2 S.sub.3 corresponding to MoS.sub.2 and WS.sub.2. The active form of vanadium sulfide thus corresponds to a non-stoichiometric composition close to trivalent vanadium.
U.S. Pat. No. 4,151,190, "Process for Producing C.sub.2 -C.sub.4 Hydrocarbons from Carbon Monoxide and Hydrogen," Craig B. Murchison and Dewey A. Murdick, gives a listing of references pertinent to the development of molybdenum and tungsten catalysts. In addition, the book Sulfide Catalysts--Their Preparation and Application, Otto Weisser and Stanislov Landa, Pergamon Press, N.Y. 1973, gives details concerning the preparation of catalysts in which one initially obtains a higher sulfide, which is subsequently converted to a lower valent modification.
In addition to catalyst preparation by impregnation on a carrier and decomposition of a solid material, catalysts of this type have sometimes been prepared by coprecipitation from organic solutions. The impregnation technique has, however, been most frequently employed though it has the disadvantage that some type of activation is usually necessary to obtain the desired active catalyst form. An additional disadvantage is the fact that the high surface area of the support does not always contribute to the production of a catalyst with a high surface area of the active components, since subsequent activation reactions change the nature and aggregation of the starting materials which have been deposited on the support.
In the case of precipitation, the preparation of the thiosalts is inconvenient and expensive since precipitation from dilute solution is required and considerable time is occupied in the process (see, for example, P. Rainasamy and A. J. Leonard, J. Cat. 26, 352 (1972). In addition, vanadium sulfide cannot be conveniently prepared by this procedure. Also it is difficult to incorporate other components into the catalyst in an intimately mixed condition.